The present invention relates to satellite positioning systems which are combined with wireless communication systems and also relates to radio location techniques.
Wireless communications are rapidly augmenting conventional telephone communications. Wireless cell telephones or cellular telephones, as they are sometimes referred to, are a form of cell based or cellular based communication systems. These systems are capable of being used in many different locations. In conventional telephone communications, emergency 911 service has been in existence for a number of years and has evolved and been upgraded over time. Currently, for most wired telephone systems, an enhanced 911 service is available. In this service, the emergency center receiving the call automatically from the telephone system learns the phone number, location and identity of the calling party without having been provided this information by the calling party. In the case of a cell telephone, such enhanced 911 service is normally not possible. At the current time, an emergency center which receives a call dialed from a cellular telephone has no idea where the party is calling from. One proposed solution to provide an enhanced 911 service for cell telephones is to use the overlay of the cells in a cell based communication system. This overlay arises from the fact that the effective radio communication distances from one cell site to the next overlap to some extent. This is shown in FIG. 1. The cell based communication system 10 includes four cells 12, 14, 16, and 18 which overlap to the extent shown in FIG. 1. Each of these cells has been drawn around its respective wireless cell based transceiver which are shown as transceivers 21, 22, 23, and 24. These transceivers transmit and receive wireless cell based communication signals to and from the wireless mobile cell based communication systems, such as the wireless cell based mobile communication systems 26, 27, and 28 shown in FIG. 1. As is well known in the art, there are numerous cell based communication systems, such as the AMPS system and the CDMA system as well as the TDMA, GSM, PCS, and the ISM systems. Each of these systems share the basic cell based communication wireless approach that consists of base sites (also referred to as cell sites) transmitting signals and deployed in the cellular topology where each cell is defined by the coverage zone of signals to and from its cell site (wireless cell based transceiver) and with some cells possibly overlapping other cells. The overlap of the cells typically will allow a radio position operation to be performed at least in the overlap areas. For example, the position of the wireless cell based mobile communication system 27 shown in FIG. 1 may be determined by determining the time of travel of communication signals between the cell site 22 and the mobile system 27 and similarly the time of travel of wireless cell based communication signals between the mobile system 27 and the transceiver 23. If certain assumptions (such as approximate altitude) are made and an angle of arrival (AOA) technique is used, the position of a mobile cell based system may be determined with as few as two wireless cell based transceivers being in radio communication with the mobile unit. Typically, however, at least three radio communication links with three different wireless cell based transceivers are required in order to obtain a two-dimension position solution. There are numerous examples in the prior art which describe the use of the cellular overlay as a way to provide radio position operations for mobile cellular based communication systems. One approach is referred to as a time of arrival (TOA) technique and another approach is referred to as a time difference of arrival (TDOA) technique.
While the infrastructure may to some extent already exist for the use of the cellular overlay to provide radio position, it turns out that the overlay is typically too small among cells in order to provide adequate coverage over the various possible positions of a mobile unit. This can be seen in FIG. 1 where the mobile unit 28 may only receive and transmit signals with one wireless cell based transceiver, the transceiver 22, and not receive signals from (or be able to transmit signals to) the other transceivers or cell sites. In this situation, the position of the wireless unit 28 can be defined no better than a circle surrounding the cell site 22 and this in fact may not be possible due to errors in the system and due to the inability to determine time accurately enough at the transmitting and receiving systems.
Another approach to determining the position of a mobile communication system is to incorporate a satellite positioning system (SPS) such as the Global Positioning System (GPS) into the same enclosure or coupled tightly with the communication system. Numerous references have described this approach in which the GPS system by itself is used to determine the position of the integrated unit. An example of such an integrated unit is described in U.S. Pat. No. 5,663,734 by Norman F. Krasner. In many such integrated systems which include a GPS receiver and a wireless communication system, the GPS receiver is a conventional correlator based receiver which often has difficulty collecting GPS signals from enough GPS satellites to determine a position of the GPS receiver. GPS receivers typically do not perform well in environments where there is blockage of the GPS signals from the GPS satellites. This blockage may be only a tree overhead, and yet the GPS receiver may not be able to acquire and track GPS satellites in such an environment. Consequently, there are many situations where an integrated GPS receiver and communication system will not be able to provide a position which could then be transmitted through the communication system back to a 911 operator at a public safety answering point (PSAP).
Another approach in the prior art which attempts to provide a position for a wireless communication system is described in U.S. Pat. No. 5,604,765. This patent discloses a technique of embedding a CDMA navigation signal into the existing wireless communication system in order to provide navigational capability. The mobile unit includes a GPS receiver and also includes a communication system which can receive GPS-like navigation signals which have been embedded into communication broadcasts from cellular and/or other wireless basestation transmitters. In this technique, a mobile system can utilize both the GPS system and the communication system. That is, the communication system, when there is blockage of the GPS satellites, may use the embedded GPS-like signals in the cellular communication signal to augment or replace the GPS signals from the GPS satellites in order to provide a position. While this technique described in U.S. Pat. No. 5,604,765 provides an advantage over a cell phone which uses merely the cellular overlay to perform radio positioning and also provides an advantage over a mobile unit which merely uses the GPS system to provide a position, this technique of embedding a GPS-like signal into the wireless cellular signals requires modifications to the existing broadcast signals and thus would require considerable modification of the infrastructure of a cell based communication system.
U.S. Pat. No. 5,327,144 describes another positioning system which uses cellular transmissions of a cellular telephone to determine the time difference of arrival (TDOA) of the transmitted signals; these time differences are obtained by using a GPS receiver to time stamp the cellular transmissions in order to measure the time differences between cellular transmissions between the cellular telephone and several cell sites. However, the GPS receiver is not used to determine pseudoranges between a GPS receiver in the cellular telephone and GPS satellites. U.S. Pat. No. 5,512,908 also describes a TDOA system which uses cellular transmissions to measure the cell phone's location from the time differences of arrival of the cellular transmissions; again, GPS signals are used in the cell sites to time stamp the signals in order to measure the time difference in signal travel time. The GPS signals are not used to determine pseudoranges between a GPS receiver in the cellular phone and GPS satellites. U.S. Pat. No. 5,612,703 describes a positioning system in a cellular communications system based on round trip signal time measurements. U.S. Pat. No. 5,724,660 describes a method for determining a position of a cellular telephone by measuring the signal strength of cellular transmissions between the telephone and cell sites; this position is then compared to a position determined from a GPS receiver which separately attempts to determine a position. Thus, this patent determines a position by comparing a position derived from signal strength measurements to a position derived from GPS measurements.
Thus it is desirable to provide a system which is capable of position determination in a variety of wireless environments and which may use both the global satellite navigation system, such as the GPS system, and a wireless system deployed in a cellular configuration.